In general, the floating support has anchor means enabling it to remain in position in spite of the effects of currents, winds, and swell. It also generally includes means for storing and processing oil and means for discharging to off-loading tankers, which call at regular intervals in order to take away the production. These floating supports are commonly referred to as floating production storage off-loading supports with the abbreviation “FPSO” being used throughout the description below.
Bottom-to-surface connections are known for an undersea pipe resting on the sea bottom, the connection being of the hybrid power type and comprising:                a vertical riser having its bottom end anchored to the sea bottom via a flexible hinge and connected to a said pipe resting on the sea bottom, with its top end tensioned by a sub-surface float to which it is connected; and        a connection pipe, in general a flexible connection pipe, between the top end of said riser and a floating support on the surface, and, where appropriate, said flexible connection pipe under the effect of its own weight taking up the shape of a diving catenary curve, i.e. going down well below the float before rising again up to the floating support.        
Bottom-to-surface connections are also known that are made by continuously raising up to the sub-surface strong and rigid pipes constituted by thick steel tubular elements that are welded or screwed together and that take up a catenary configuration of continuously varying curvature all along their suspended length, commonly referred to as steel catenary risers (SCRs) and also commonly referred to as rigid catenary risers.
Such a catenary pipe may rise up to the support floating on the surface, or it may rise no further than a sub-surface float that tensions its top end, which top end is then connected to a floating support by a diving flexible connection pipe. Catenary risers of reinforced configuration are described in WO 03/102350 in the name of the Applicant.
In WO 00/49267, SCR rigid pipes are proposed as connection pipes between the floating support and the riser having its top tensioned by a float immersed below the surface, and the float is installed at the head of the riser at a greater distance from the surface, in particular at least 300 meters (m) from the surface, and preferably at least 500 m.
WO 00/49267, in the name of the Applicant, describes a multiple hybrid tower including an anchor system with a vertical tendon constituted either by a cable or by a metal bar or even by a pipe that is tensioned at its top end by a float. The bottom end of the tendon is fastened to a base resting on the bottom. Said tendon includes guide means distributed along its entire length with a plurality of said vertical risers passing therethrough. Said base may merely be placed on the sea bottom and remain in place under its own weight, or it may be anchored by means of piles or any other device suitable for keeping it in place. In WO 00/49267, the bottom end of the vertical riser is suitable for being connected to the end of a bent sleeve that is movable relative to said base between a high position and a low position, said sleeve being suspended from the base and being associated with return means that urge it towards a high position in the absence of a riser. This ability of the bent sleeve to move enables variations in riser length under the effects of temperature and pressure to be absorbed. At the head of the vertical riser, an abutment device secured thereto bears against the support guide installed at the head of the float and thus keeps the entire riser in suspension.
The connection with the undersea pipe resting on the sea bottom is generally provided via a portion of pipe having a pigtail shape or an S-shape, said S-shape being made in a plane that is either vertical plane or horizontal plane, the connection with said undersea pipe generally being made via an automatic connector.
Thus, a wide variety of bottom-to-surface connections are in existence that enable undersea well heads to be connected to a floating support of the FPSO type, and in certain oil field developments, a plurality of well heads are connected in parallel to a common bottom-to-surface connection so as to limit the extent to which the side of the FPSO is occupied, since each of said bottom-to-surface connections must be spaced apart from its immediate neighbors so as to avoid any interference and any impacts, not only between the floats, but also between the flexible pipes and electric cables connecting with said FPSO.
In certain oil field developments, it is necessary to connect each of the well heads individually to a said FPSO, and there are thus very many bottom-to-surface connections and it is not possible to install all of them because the length of the side of the FPSO is limited and can accept only a limited number of bottom-to-surface connections.
It is desired to use as many bottom-to-surface connections as possible from a single floating support in order to optimize the exploitation of oil fields. That is why various systems have been proposed for enabling a plurality of vertical risers to be associated together in order to reduce the size of the exploitation fields and in order to be able to use as many bottom-to-surface connections as possible connected to a common floating support. Typically, it is necessary to make provision for installing up to 30 or even 40 bottom-to-surface connections from a common floating support.
WO 00/49267 describes a multiple hybrid tower including an anchor system with a vertical tendon constituted either by a cable or by a metal bar or even by a pipe that is tensioned at its top end by a float. The bottom end of the tendon is fastened to a base resting on the bottom. Said tendon includes guide means distributed along its entire length with a plurality of said vertical risers passing therethrough. Said base may be merely placed on the sea bottom and remain in place under its own weight, or it may be anchored by means of piles or any other device suitable for keeping it in place. In WO 00/49267, the bottom end of the vertical riser is suitable for being connected to the end of a bent sleeve that is movable relative to said base between a high position and a low position, said sleeve being suspended from the base and being associated with return means that urge it towards a high position in the absence of the riser. This ability of the bent sleeve to move enables variations in riser length under the effects of temperature and pressure to be absorbed. At the head of the vertical riser, an abutment device secured thereto bears against the support guide installed at the head of the float and thus keeps the entire riser in suspension.
The connection with the undersea pipe resting on the sea bottom is generally provided via a portion of pipe having a pigtail shape or an S-shape, said S-shape then being made in a plane that is either vertical or horizontal, the connection with said undersea pipe generally being made via an automatic connector.
That embodiment comprising a multiplicity of vertical risers held by a central structure having guide means is relatively expensive and complex to install. Furthermore, the installation needs to be prefabricated on land prior to being towed out to sea, and then once on site up-ended in order to be put into place. In addition, maintenance thereof also requires relatively high operating costs.
In WO 02/66786 and WO 02/103153, in the name of the Applicant, multiple-riser hybrid towers are described having vertical riser anchor systems suitable for receiving two risers side by side from a common anchor base, with the floats at the heads of said risers being fastened and secured to each other by means of a hinged parallelogram structure. The two risers are also connected together by tubular collars fastened to one of the risers and connected by rings that slide freely around the second riser, such that the two risers can follow substantially the same lateral movements while being relatively more independent of each other in their vertical movements.
When it is desired to associate a plurality of risers with a common floating support, the problem arises of interface between the movements of said risers that are subjected to the same movement as their tensioning float at the top under the effect of movements of the floating support at the surface, which is subjected to swell, wind, and current.
When a multiplicity of bottom-to-surface connections of the hybrid tower type are implemented, each comprising a single vertical riser, it is necessary in practice for the various connections to be spaced apart from one another, for at least the following two reasons:
1) all of the respective bases of the two hybrid towers when anchored via suction anchors anchored to the sea bottom must be spaced apart by distances of not less than five times and preferably at least ten times the diameter of said anchors in order to avoid interference in terms of secure connection to the sea bottom and in order to guarantee reliable anchoring; and
2) secondly, the floats at the tops of the risers are subjected to movements within a cone having its apex situated at the anchor system, and of an angle that makes it necessary to provide sufficient distance between the various floats at the tops of the vertical risers in order to prevent them from striking against one another.
Those constraints involve spreading out the exploitation zone and limiting the number of bottom-to-surface connections that can be connected to a common floating support, via the sides thereof, in order to avoid interference between the various connections.
Furthermore, since the crude oil is conveyed over very long distances, i.e. several kilometers, it is necessary to provide an extremely expensive level of insulation, firstly to minimize any increase in viscosity that would lead to a drop in the hourly production rate from the wells, and secondly to avoid the flow becoming blocked by paraffin being deposited or by hydrates forming when the temperature drops to around 30° C. to 40° C. These phenomena are critical because the temperature at the bottom of the sea is about 4° C. and, particularly in West Africa, the crude oils are of the paraffin type. It is therefore desirable for bottom-to-surface connections to be short in length and thus for the space occupied by the various connections to a common floating support to be limited.
That is why it is desirable to provide an installation suitable for enabling a common floating support to operate a plurality of hybrid tower type bottom-to-surface connections that occupy a limited amount of space with limited movement, and that are also simple to lay, with it being possible for them to be fabricated at sea on board a pipe-laying ship, in order to avoid prefabrication on land followed by towing out to site and upending in order to put the installation finally into place.